Field of the Invention
The present disclosure relates to a method for producing a highly reliable multilayer ceramic capacitor such as a multilayer ceramic capacitor for industrial equipment.
Description of the Related Art
In recent years, multilayer ceramic capacitors have been increasingly used in industrial equipment such as automobiles as well as consumer electronics such as communications devices. Such multilayer ceramic capacitors are required to be highly reliable.
An example of a multilayer ceramic capacitor having such uses is disclosed in Japanese Unexamined Patent Application Publication No. 2014-165447. The multilayer ceramic capacitor disclosed in this document includes BaTiO3-based dielectric ceramic layers. These dielectric ceramic layers have 4.2×1012 or less oxygen vacancies per mm3.
The document states that this multilayer ceramic capacitor does not have many carriers (oxygen vacancies) which move therein in response to application of a DC voltage and thus is highly insulating and has a high dielectric strength.
The following is a description of the studies conducted by the inventor to accomplish the present disclosure.
In order to improve the reliability of a multilayer ceramic capacitor which includes BaTiO3-based dielectric ceramic layers, it is necessary to suppress the movement of oxygen vacancies within the dielectric ceramic layers during application of a DC voltage. In this description, the term “reliability” denotes the time taken for insulation resistance to decrease to a certain level in a high-temperature loading test (this time taken hereinafter may be referred to as lifetime under high-temperature loading). It seems that an effective way to suppress the movement of oxygen vacancies is to replace Ba ions in the A sites (12-fold coordination surrounded by O2− ions) of the BaTiO3 crystal lattice with rare-earth metal ions such as Dy ions or Ho ions.
The replacement of Ba ions having a charge of +2 with rare-earth metal ions having a charge of +3 creates excess positive charges. As a result, Ba vacancies, which appear to have a relative charge of −2, are formed to compensate the positive charges. These Ba vacancies form stable defect pairs with the oxygen vacancies which appear to have a relative charge of +2. Since the Ba vacancies do not move easily even when a DC voltage is applied, the oxygen vacancies captured by the Ba vacancies also do not move easily.
That is, the reliability of a multilayer ceramic capacitor including BaTiO3-based dielectric ceramic layers depends on how many rare-earth metal ions have entered the A sites of BaTiO3. It seems here that the rare-earth metal ions can enter the A sites by replacing the Ba vacancies already existing in the crystal lattice more easily than by pushing out the Ba ions of BaTiO3.
Assuming that rare-earth metal ions cannot easily enter the A sites of BaTiO3 unless the A sites have Ba vacancies, when a dielectric ceramic material containing a rare-earth metal compound is sintered, the rare-earth metal compound in the dielectric ceramic material becomes localized in the grain boundaries of the resulting sintered dielectric ceramic layers. When a DC voltage is applied across these dielectric ceramic layers, the electric field may concentrate locally in the position of the rare-earth metal compound. That is, too small a number of Ba vacancies may cause localization of the rare-earth metal compound and this may result in a reduction in reliability.
On the other hand, in the case where the number of Ba vacancies is too high in the BaTiO3 which is the base material for the dielectric ceramic layers, the BaTiO3 particles become too active during sintering of the dielectric ceramic layers and are readily “necked” together. This leads to easy grain growth and, as a result, the ceramic structure of the sintered dielectric ceramic layers may become uneven. When a DC voltage is applied across these layers, local concentration of the electric field may occur due to the unevenness of the ceramic structure. That is, too many Ba vacancies may cause unevenness in the ceramic structure and this may result in a reduction in reliability.
As has been described, it appears that the Ba vacancies in the BaTiO3 which is the base material for the dielectric ceramic layers affect the reliability of a multilayer ceramic capacitor. However, the foregoing document provides no mention that the Ba vacancies in the BaTiO3 which is the base material for the dielectric ceramic layers significantly affect the reliability of a multilayer ceramic capacitor.